Because of the low energy levels of decay particles emitted from tritium when compared to those emitted from other radioisotopes, standard radioactivity detection techniques, such as Geiger counters, are generally inadequate determining the presence and concentration of tritium on a surface. Because of this, a variety of techniques and devices have been developed for the detection of tritium on surfaces.
The most commonly used method for the measurement of removable tritium surface contamination is wipe testing, which is also known as smearing. In this method, a standard filter disc-type wipe, made of polystyrene or paper, is rubbed over a known surface area (typically 100 cm.sup.2) of the surface to be monitored. The wipe is then analyzed for tritium, usually with liquid scintillation counting or a windowless proportional counter. While this technique is relatively simple to perform, it does have significant drawbacks. Foremost, this technique only measures a fraction of the removable surface contamination. Also, the measured concentration of tritium can vary significantly due to the vagaries associated with wiping of surfaces, such as the type of wipe media, the nature of the surface and the manner in which the wiping is performed. Further, this technique does not afford real time monitoring of tritium contamination, tends to alter the surface and labour intensive to perform.
Windowless plastic scintillators have also been used to measure tritium contamination of surfaces. These devices comprise a flat plastic scintillator which is viewed by two photomultipliers detecting coincident-light photons. With these scintillators, it is necessary to achieve a seal between the scintillator and the surface that is capable of both excluding external light and sustaining a partial vacuum. As such a seal can be difficult to achieve in field usage, these devices have, for practical purposes, generally been limited to laboratory use.
Windowless proportional gas flow counters have also been used to detect tritium surface contamination. These devices contain a multiwire proportional counter with mesh cathodes and require a high potential and the supply of a counting gas. Because of the need to supply a counting gas, and of the fragility of the collecting wires, the use of these devices has been limited.